The Army Corps of Engineers guy on WLL speculated that the waves were very strong against one wall of the canal, then bounced back across the canal where the breach was made and eroded the levee. (Does that make sense? My interpretation of what he said.)
The canal acted as a waveguide, which reflected and amplified the wave action.
You can hear the audio analog if you set up a sound source, stick a cardboard tube to your ear and point it at the sound source. The sound sounds much louder through the tube because the reflections "amplify" the noise.